722 



SUBANTARCTIC ISLANDS OF NEW ZEALAND. [Physiografhy , Geology. 



ship. Biotite occurs freely, and in the coarse varieties it is an important constituent. 

 It is usually in ragged and irregular forms, moulded on the feldspar, and forming 

 zones round the olivine and iron-ores, but frequently numerous flakes are included 

 in the pyroxene. Enstatite appears occasionally, especially in the coarser types. 

 It was not seen in sections from the boulders at Camp Cove, but it appears in speci- 

 mens taken from the solid rock. Magnetite occurs freely in irregular masses and 

 square-sided grains ; apatite is also present. 



The composition of one of the coarser specimens is shown in the following com- 

 plete analysis by Mr. A. M. Wright : — 



Per Cent. 



SiOa . . . . . . . . . . . . 49-44 



11-38 



AI2O3 



¥e,0, 



FeO 



MnO 



CaO 



MgO 



K2O 



NaaO 



P2O5 

 TiO,, 



CO/ 



Water lost below 100 



C. (H2O-) 

 above 100° C. (HgO-f) 



2-41 



12-53 



0-32 



7-30 



12-26 



1-19 



2-91 



0-12 



0-72 



Trace 



0-12 



Trace 



100-70 



A silica determination of another sample of rock gave a result as low as 42-44, 

 which no doubt belongs to an exceptionally basic variety. The specific gravity of 

 a number of specimens was found to range from 3-02 up to 3-23. 



One interesting feature of the gabbro is its association with a marked magnetic 

 disturbance. The magnetic records show that the general declination of the needle 

 in the islands is about 18° E., while in the Carnley area the following readings occur : 

 Adams Islands, 18° ; Camp Cove, 17° 46' ; Coleridge Bay, 16° 21' ; McClure Head, 

 7° 49' ; North Arm, 20° ; Norman's Inlet, 18° 32'. 



The marked disturbance will be seen to coincide exactly with the known occur- 

 rence of gabbro in position, or with the close proximity of it. Similar phenomena 

 are associated with the gabbro in the west of Scotland.* 



The special features of this case seem to be the normal behaviour of the com- 

 pass on a highly ferriferous basalt, but its abnormal behaviour when near a gabbro 

 which is overlaid by the same basalt. 



In the present state of our knowledge nothing further can be said about this 

 disturbance, but some light may be thrown on it when the final results of the mag- 

 netic survey of New Zealand are fully discussed in connection with the geological 

 structure of the country. 



* Harker, " Tertiary Igneous Bocks of Skye." 



